fm10k: update function header comment for fm10k_get_stats64

Re-word the comment to avoid stating that we return a value for this
void function. Additionally, there is no need to mention older kernels,
since this is the upstream kernel.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Krishneil Singh <krishneil.k.singh@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>